A. Field of the Invention
This invention relates generally to machine vision methods and apparatus for locating two dimensional objects that are not aligned to a camera.
B. Background
Machine vision systems are typically used to locate and measure objects being fabricated or inspected in some assembly operation. For example, semiconductor devices, such as integrated circuit chips, are electrically connected to leads on a lead frame. This involves placing and connecting a wire to electrically connect a pad residing on a die (semiconductor chip) to a lead in a lead frame.
Machine vision tools for gauging have been used to locate leads on such a device, and report lead width and lead spacing so the bonding devices will work more accurately. Gauging tools, often called caliper tools, are modeled after mechanical calipers. In a typical machine vision caliper operation, a user specifies the desired separation between the caliper "jaws" or the approximate distance between the edge pairs of interest such as integrated circuit leads.
In a typical application, integrated circuit leads may need to be found for alignment of a wire bonder. In a wire bonder, pads on the semiconductor die are connected to leads using a thread-like wire usually made of gold or aluminum. The wire typically follows a straight path from the lead to the pad. In an inspection system, the wire needs to be inspected for deviation from a straight line which represents its ideal location. When viewed from above the plane of the semiconductor dies, the amount of deviation of the wire from a straight line is known as the wire curl. Determining whether or not a wire is present and intact at all is called determining wire presence.
In a wire bonding application, a post-process inspection step, commonly called the third optical inspection, involves locating and inspecting the position and size of all bonds on the device, the wire connections, wire presence, and the wire heights using optical means to insure that a good connection was created by the bond. This is done after wire bonding is complete and before encapsulation of the integrated circuit.
Machine vision systems or image processing systems (systems that capture images, digitize them and use a computer to perform image analysis) are used on wirebonding machines to align devices and guide the machine for correct bonding placement, but have heretofore not been used during the process for inspection purposes. Separate machines are available to perform inspections outside of and after the bonding process, offline but this requires another piece of capital equipment in the production line, and many inspections cannot be done without information that was available while the bonder was operating.
Heretofore, there appeared to be no known methods of automatically determining wire presence or measuring wire curl, either as a post-process automatic inspection or as an online inspection. If the wires were all aligned to the camera, and treated as two-dimensional objects to be dimensionally gauged, a caliper-like vision tool might be considered, but caliper-like vision tools are optimized to work fastest when the objects are perfectly aligned to the camera's vertical or horizontal axis. If the object is not aligned in some cases the measurement cannot be made, or, the measurement is made with low accuracy or with a different procedure or either the object or vision tool must be rotated to align with one another. Two-dimensional objects to-be-measured often can appear in any orientation in front of the camera, rotated at any angle.
Two previous approaches have been used to find ways to apply caliper-like tools to the more generalized problem of the nonaligned two-dimensional object: the first, uses linear interpolation to rotate images to align objects to the camera and the second, uses skewed windows shaped like parallelograms that are aligned to the objects. The drawback of using linear interpolation is speed. It is too slow to meet certain kinds of application needs.
The drawback of using skewed windows for some applications is that the final results of applying a caliper-like vision tool change with the changing angle of the object. That is, the measurement of the same object may be different depending on its orientation. There are a limited number of applications where this change is acceptable.
Finally, in order to locate a wire automatically during the wirebonding assembly process to perform an inspection, the speed of the vision tools used is a critical factor. Wire presence in a wire bonding inspection is presently measured by manual means using a microscope after the die has been bonded. In most applications, the inspection is done on a sampled basis, but for expensive parts, it may be necessary to inspect 100% of the parts being bonded. Manual techniques for this are both costly and time-consuming.
Since wires are specular three dimensional objects, their appearance changes drastically with very small changes in source illumination direction. Wires usually pass above a background of a semiconductor die, a wire bonder heating plate, and a lead frame. Techniques for locating and inspecting the wire with respect to such a confusing background need to be robust as well as fast and are consequently more difficult to develop.
In addition, the optical system of the image processor or machine vision system is usually focused on either the semiconductor die or the lead frame but not the wire, so the wire itself is not in focus. Processing out of focus images is also more difficult.